1. Field of the Invention
The invention relates to a latent image structure, for example for use as a security device.
2. Description of the Related Art
A latent image is an image which cannot be seen when viewed normally (i.e. perpendicularly) but can be seen when the surface carrying the image is held at an acute angle to the eye. Conventional latent images are formed as relief structures which have sufficient height to mask certain regions when the structure is held at an acute angle. These relief structures can be form(ed by embossing or printing, typically intaglio printing. A number of different types of intaglio printed latent images are described in U.S. Pat. No. 4,033,059. In addition, this document discloses complex, multiple latent images which enable more than one latent image to be viewed when the structure is viewed at different angles. This is particularly useful where the structure is to be used as a security device.
For example, in U.S. Pat. No. 4,033,059, two superimposed latent images are described, each being formed by continuous lines. Where the two structures overlap, a different line structure is used which means that when the features are viewed from a shallow angle, the strength of the image will vary according to whether or not the image forms part of the second image area. In another form, a set of dots is used. A regular dot structure forms a bulk of the feature area with strategically placed dots so produce images in the required direction resulting in a combination of latent and transient images. This means that the presence of an image is easily seen at normal viewing and furthermore a shadow latent image only can be produced.
A problem which can arise with latent images is that although they are primarily visible only when viewed at an acute angle, nevertheless they require different line structures which are at least partially visible when the device is viewed normally. This is true, for example, of the structures described in U.S. Pat. No. 4,033,059 and reduces the security nature of the device. This problem becomes even more noticeable with multiple latent images.